[20.05] The Edge of the Solar System

R. L. Allen, G. M. Bernstein (U.Mich), R. Malhotra (LPI)

The population of the Kuiper Belt within 50~AU of the Sun
has likely been severely depleted by gravitational
perturbations from the giant planets, particularly Neptune.
The density of Kuiper Belt objects might be expected to be
higher just beyond 50 AU, where planetary perturbations are
very weak. In 1998 and 1999, we surveyed for Kuiper Belt
Objects (KBOs) in 6 fields of the ecliptic (total sky area
1.4 deg2) with limiting magnitudes between R=25.0 and
R=25.8. This is deep enough to detect KBOs of diameter
\gtrsim 160~km at a distance of 65~AU. We discovered 24
objects. None of these objects, however, is beyond 55~AU.

Our survey places a 95% CL upper limit of \Sigma < 5\,{\rm
deg}-2 on the surface density of KBOs larger than
~60~km beyond 55~AU. This can be compared to the
surface density of ~\,{\rm deg}-2 of \ge160~km
KBOs at distances 30---50~AU determined from this survey and
previous shallower surveys. A comparison of the volume
densities indicates the region from 55---65~AU has a volume
density at most 72 40---50~AU, at a 95 50~AU is excluded in
this model-independent estimate, implying that some process
or event in the history of the Solar System has truncated
the distribution of 200-km planetesimals at ~0~AU. A
dense primordial disk could be present beyond 50~AU if it
contains only smaller objects, or is sufficiently thin and
inclined to have escaped detection in our 6 survey fields.

This work is supported by NASA Planetary Astronomy grant
#NAG5-7860.

The author(s) of this abstract have provided an email address
for comments about the abstract:
rhiannon@umich.edu